1. Field of the Invention
This invention relates to an automatic electronic component-mounting apparatus in which a mounting head is lifted/lowered to thereby pick up electronic chip components by vacuum and mount the components on a printed circuit board.
2. Prior Art
Conventionally, an automatic electronic component-mounting apparatus is known in which each electronic chip component is picked up from a component-feeding device by a vacuum nozzle arranged on a mounting head and mounted on a printed circuit board (as disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai;) No. 5-192824).
However, in the above electronic component-mounting apparatus, since each mounting head is mounted on an outer periphery of a rotary table via a guide rod having the mounting head attached thereto and extending through the rotary table, the rotary table is required to have a large radial dimension, which makes it impossible to reduce the size and weight of the apparatus. In mounting the mounting head on the rotary table, a ball holding-type linear guide (e.g. CAT-1777 or CAT-1770 produced by Nippon Thompson Co. Ltd.) can be employed. However, if the CAT-1777 type linear guide is used, the radial dimension of the rotary table is also required to be large, and besides, rigidity of the rotary table is not high enough. On the other hand, if the CAT-1770 type linear guide is used, adjustment by a setscrew for setting a preload is required. This makes the setting of the preload troublesome and difficult, and hence adjustment for the assembly takes too long time.
To overcome the above problems, there was proposed a technique by Japanese Laid-Open Patent Publication (Kokai) No. 8-195582, in which an electronic component-mounting apparatus is provided with a head-lifting/lowering mechanism having a pair of rails and a nut member which is slightly larger in thickness than the pair of rails and slidably arranged between the pair of rails, the nut member of the head-lifting/lowering mechanism being secured to the rotary table and the pair of rails being secured to a head-lifting/lowering member.
In the above device, when tact time of mounting each electronic component is reduced, i.e. when the cycle of operation of the head-lifting/lowering mechanism is shortened, the mounting head is lifted/lowered at high speed, with fast sliding of the nut member over the rails. As a result, the rails, the nut member, and the head-lifting/lowering member are heated to a high temperature. The nut member and rails are required to be sufficiently rigid for serving as a bearing, so that in general, they are formed of stainless steel. On the other hand, the head-lifting/lowering member, which is required to be light, is formed of aluminum. Therefore, if the members are excessively heated, a difference in the coefficient of thermal expansion between stainless steel and aluminum causes a considerable difference in thermal expansion between the nut member and rails and the head-lifting/lowering member, which hinders the speed of mounting of electronic components from being increased.
More specifically, since stainless steel has a smaller coefficient of thermal expansion than that of aluminum, the rails and the head-lifting/lowering member are deformed due to the bimetal effect. Further, since the head-lifting/lowering member is secured to the pair of rails, a distance between the pair of rails becomes larger than a width of the nut member, and hence gaps between the respective rails and the nut member expand. As a result, a preload applied by balls interposed between the nut member and the respective rails is decreased or even lost.
However, if the head-lifting/lowering member were also formed of a ferrous material, such as stainless steel, so as to avoid the above inconvenience, the weight of the head-lifting/lowering member would be increased, which also hinders high-speed mounting of electronic components.
It is an object of the invention to provide an automatic electronic component-mounting apparatus, which is capable of mounting electronic components at high speed without being affected by thermal expansion of the relevant components.
To attain the above object, the present invention provides an automatic electronic component-mounting apparatus including a main block, a pair of rails, a nut member which is slidably arranged between the pair of rails, a head-lifting/lowering member which is lowered and lifted with respect to the main block via the pair of rails and the nut member, and a mounting head attached to the head-lifting/lowering member, the head-lifting/lowering member being lowered and lifted to cause the mounting head to pick up an electronic chip component and then mount the electronic chip component on a printed circuit board.
The electronic component-mounting apparatus according to the invention is characterized in that the head-lifting/lowering member is formed of a metal-ceramic composite material produced by combining a ceramic reinforcing material with a metal.
According to this automatic electronic component-mounting apparatus, even when the rails and the nut members are formed of a ferrous material, such as stainless steel, which has a high strength, it is possible to provide a head-lifting/lowering member which has a coefficient of thermal expansion close to that of the ferrous material and is light in weight.
Preferably, the metal is aluminum or an aluminum alloy.
More preferably, a volume fraction of the ceramic reinforcing material is approximately 30%.
According to these preferred embodiments, it is possible to make the thermal expansion coefficient of the head-lifting/lowering member close to that of the ferrous material such as stainless steel and avoid degradation of machinability of the head-lifting/lowering member.
Preferably, the nut member is slightly larger in thickness than the pair of rails and is secured to the main block, and the pair of rails are secured to the head-lifting/lowering member.
According to this preferred embodiment, adjustment of a preload set by balls interposed between the rails and the nut member is facilitated, which contributes to reduction of time required for adjustment of the assembly.
Preferably, the main block comprises a rotary table, and a plurality of the head-lifting/lowering members arranged on a periphery of the rotary table.
According to this preferred embodiment, since the head-lifting/lowering members, which conventionally readily undergo thermal expansion when heated due to their frequent vertical sliding motions, are no longer susceptible to the problem of thermal expansion, it is possible to carry out the mounting of components at a high speed.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.